It is well known that in the production of steel, especially strip steel, in order to improve the quality of the surface of the steel, it is subjected to treatment with a solution of hydrochloric acid. This step is called “pickling”, and the resultant liquor from this step containing ferrous chloride FeCl2 is called “waste pickle liquor” (WPL). In general, the waste pickle liquor from such a treatment comprises an aqueous solution of about 3-5% HCl and 15-20 weight percent of ferrous chloride. In rare occasions, the WPL may comprise ferrous sulfate FeSO4 in case the pickling is performed using sulfuric acid H2SO4. Ferrous sulfate may also arise as a byproduct of Ti-production.
Several methods have been proposed for economically treating the pickle liquor in order to dispose of same and/or for converting the liquor to an easily disposable and/or useful product, e.g. ferric chloride (FeCl3). For example, one process involves treating the pickle liquor with chlorine gas in order to convert the ferrous chloride to ferric chloride. However, chlorine gas Cl2(g) is not always available and requires the presence of a production unit, e.g. an electrolysis unit. If Cl2 would be supplied in liquid phase, then cost, toxicity and safety problems usually arise. Oxygen O2, hydrochloric acid HCl in liquid phase and sulfuric acid H2SO4 can be made more easily available in large quantities as they are routinely transported.
US application US 2003/211031 discloses a process for producing ferric chloride whereby pickle liquor containing ferrous chloride and fortified with sufficient HCl is converted to ferric chloride in the presence of oxygen in a tower at a temperature above 132° F. The ferric chloride solution from the tower is subjected to evaporation so as to increase the concentration of the ferric chloride. The resultant concentrate is recycled into the tower until a concentration of about 40% by weight ferric chloride is obtained. A gas phase from either or both of the tower and the evaporator is scrubbed in order to remove HCl which can be used to fortify the pickle liquor. Flow rates are 1000-1500 gallons/min.
Japanese application JP 2002-001365 discloses a simple method of recycling an etching waste liquid by defects in the conventional ejector method for recycling iron chloride from etching waste water. A ferrous chloride solution is hereby oxidized by supplying oxygen containing gas using an ejector mechanism, whereby the free hydrochloric acid concentration in the oxidation reaction liquid is maintained at 1 mass % or less, to manufacture a ferric chloride solution. The method disclosed in this application seems to work only at high temperatures and at slow reaction rate.
U.S. Pat. No. 5,489,380 discloses an apparatus and a process for the biological purification of sewage, with which sewage, containing dissolved pollutants, and air are fed together to a reactor via at least two mutually separate nozzles. To achieve a high substance exchange in the reactor, the streams of the two-substance mixture emanating from the nozzles are conducted such that they make impact with each other in the said reactor in an impact zone. The sewage is passed from the reactor into Pa settling tank, in which bio sludge settles. This document does not disclose the possibility of chlorinating or oxychlorinating ferrous iron in a solution to ferric iron. In fact, this document relates to the treatment of sewage with air or pure oxygen and not to the oxidation, chlorination or oxychlorination of ferrous iron. As U.S. Pat. No. 5,489,380 is concerned with purification of sewage, rest gasses which are formed after reacting with the sewage are not re-introduced as reagens into the reactor, but are extracted from the process.
There remains a need in the art for an improved method and system for oxidizing metal ions, in particular for oxidizing ferrous iron Fe(II) to ferric iron Fe(III), which may be operated both in continuous and in batch mode and preferably in continuous mode, with improved reaction rates, at lower temperatures than state of the art techniques and at or near atmospheric pressure, and which has better scalability properties, i.e. wherein the production rate can be increased more efficiently by scaling up the installation than in the case of scaling up prior art techniques.
The present invention thereto aims to provide a method and system with an increased oxidation rate of a metal ion such as ferrous iron to ferric iron as compared to prior art methods and systems, which furthermore comprises improved scalability and versatility in production rate and operation parameters, which is further capable of oxidizing ferrous iron at atmospheric pressure or about atmospheric pressure if desired, and in a continuous way.
An objective of the present invention is not only to provide an improved process for treating pickle liquor, but also to provide a process for the conversion of ferrous iron to ferric iron, and in particular of:                ferrous chloride to ferric chloride;        ferrous sulfate to ferric chloro-sulfate; and/or        ferrous sulfate to ferric sulfate,irrespective of the source of the ferrous iron. Another object is to provide a system to conduct the process.        